1. Field of the Invention
The invention concerns methods and devices for a SAR monitoring for a magnetic resonance tomography apparatus.
2. Description of the Prior Art
Magnetic resonance tomography apparatuses are known from DE 10 2005 052 564, for example.
In the case of operation of MR scanners with a transmission (TX) array and with the generation of RF pulses that can exhibit an arbitrary pulse shape for each array element (variation of amplitude and phase), a multitude of possibilities results for overlaying the electrical fields in the body of the examination subject. A high level of complexity exists if the local SAR should be comprehensively monitored or calculated in advance for parallel pulses (simultaneously emitted pulses). Monitoring of the local SAR value, however, is absolutely necessary for the safety of the patient and is required by corresponding regulations. The overlaying of the electrical fields in array antennas is particularly critical because the E-vectors (electrical field vectors) add linearly but the local power that is released (applied) by a pulse is proportional to E2.
However, because the local SAR is not directly measurable, it is necessary to rely on generating suitable body models representing the (complex) conductivity distribution and calculating in these models the fields that are produced by the respective array elements and the model points of the model. At present such calculations are commonly implemented using a technique known as the FDTD (Finite Differential Time Domain) method.
For a typical body mode, these calculations presently require multiple hours and may possibly be accelerated by the use of special processors. An element of the sensitivity matrix Skl with the property Ekl=Skl*lk arises for each TX array element k and every model point in the body model l as a result of the FDTD calculation.
In order to be able to calculate the local heat generation at specific model points for a particular point in time, the fields of the individual antennas are added for the parallel transmission pulses that occur at this point in time, and then the released electrical power density is calculated. In the case of isotropic conductivity σ, this is the term Re(σE*Ē). In the case of anisotropic conductivity, a conductivity tensor occurs at the point of the scalar σ.
The heat released at a specific time interval then results as the time integral of these powers. If this is calculated as a discrete sum, it must be taken into account that the amplitudes and phases of the individual currents can change significantly for known parallel transmission pulses within 10 ms, and therefore the sampling points in time must lie correspondingly close together. Generally, discrete RF pulses are repeated multiple times in each phase sequence for MR imaging; the SAR values accumulated over time then result as a summation across the SAR contributions of the individual pulses.
If it is sought to monitor the model precisely in its entirety, such a calculation would have to be conducted at every model point. The procedure seeks to combine adjacent model points with similar conductivities, but the problem remains that, due to the multiple overlaying possibilities of respective amplitudes and phases of the Nchannels (Nchannels can typically be 8), there is generally not a single “hotspot” in the body at which the electrical power density is always greatest. Under the circumstances it is consequently necessary to incorporate a number of positions to determine the local SAR.
An analogous problem is posed with regard to the global power loss if the total energy that is deposited in the patient volume is to be calculated from the time-dependent vector of the antenna voltages and a previously measured conductance matrix or, respectively, scatter matrix of the system.
From ISMRM 2008, Abstract #74, Graesslin et. al., an approach is known to calculate the electrical power densities online for numerous model points on the basis of sensitivity matrices calculated in advance, with the aid of fast parallel processors. An approach for parallel excitation with an array of transmission coils is known from Zhu, “Parallel Excitation With an Array of Transmit Coils”, MRM 51:775-784 (2004).